Twelve > Parabola
Asked by Atith Adhikari · 2 years ago

For the parabola x2 - 4x - 3y + 13 = 0 find:

  1. the coordinates of the vertex
  2. the coordinates of the focus
  3. equation axis of the parabola
  4. equation of directrix
  5. length of latus rectum
  6. the coordinates of the ends of the latus rectum

visibility 0
chat_bubble_outline 1
Atith Adhikari Atith Adhikari · 2 years ago
Verified

Solution

The given parabola $\rm x^2 - 4x - 3y + 13 = 0$ follows the following equation of parabola:

$$\rm (x - h)^2 = 4a (y-k)$$

where (h,k) represents the vertex of the parabola.

To find the coordinates of the vertex, we factorize the given equation of the parabola

$\rm x^2 - 4x - 3y + 13 = 0$

$\rm or, x^2 - 4x + 13 = 3y$

$\rm or, x^2 - 4x + (4 - 4) + 13 = 3y$

$\rm or, x^2 - 4x + 4 + 9 = 3y$

$\rm or, x^2 - 2 \cdot 2 \cdot x + 2^2 = 3y - 9$

$\rm or, (x - 2)^2 = 3 (y - 3)$

$\rm or, (x - 2)^2 = \frac{4}{4} \cdot 3 \cdot (y - 3)$

$\rm \therefore (x - 2)^2 = 4 \cdot \frac{3}{4} ( y - 3)$

Now, we compare this equation with the above equation to get the values of (h,k). So, $\rm (h, k) = (2,3)$.

Also, $\rm a = \frac{3}{4}$.

The coordinates of the focus are given by (h, k + a). So, the coordinates of the focus is

$\rm (h, k+a) = \left ( 2, 3 + \frac{3}{4} \right )$

$\rm = \left ( 2 , \frac{12 + 3}{4} \right )$

$\rm = \left (2, \frac{15}{4} \right )$

The equation of the axis of the parabola is given by

$\rm x= h$

$\rm \therefore x = 2$

The equation of the directrix of the parabola is given by

$\rm y = k - a$

$\rm or, y = 3 - \frac{3}{4}$

$\rm or, y = \frac{12 - 3}{4}$

$\rm or, y = \frac{9}{4}$

$\rm or, 4y = 9$

$\rm \therefore 4y - 9 = 0$ is the required equation of the directrix of the parabola.

The length of the latus rectum is given by $\rm 4a = 4 \cdot \frac{3}{4} = 3$ units.

We know, that the latus rectum passes through the focus of the parabola. For the given parabola, the y-coordinates of the focus are the same as the y-coordinates of the ends of the latus rectum.

Let us assume that the point P $\rm (x_1, k+a)$ and $\rm (x_2, k+a)$ are the coordinates of the ends of the latus rectum. We know, $\rm k + a = \frac{15}{4}$.

Put the value of $\rm y = k + a$ in the equation of the parabola to get the two values of x, we get,

$\rm (x - 2)^2 =3 (y - 3)$

$\rm or, (x - 2)^2 = 3 \left ( \frac{15}{4} - 3 \right )$

$\rm or, (x - 2)^2 = 3 \cdot \left ( \frac{15 - 12}{4} \right )$

$\rm or, (x - 2)^2 = 3 \cdot \frac{3}{4}$

$\rm or, (x - 2)^2 = \frac{9}{4}$

$\rm or, (x - 2) = \pm \sqrt{ \frac{9}{4} }$

$\rm or, x - 2 = \pm \frac{3}{2}$

Taking positive sign, we get,

$\rm or, x - 2 = \frac{3}{2}$

$\rm or, x = \frac{3}{2} + 2$

$\rm or, x = \frac{3 + 4}{2}$

$\rm \therefore x= \frac{7}{2}$

Let this value of x be the value of $\rm x_1$. So, $\rm x_1 = \frac{7}{2}$.

Taking negative sign, we get,

$\rm or, x - 2 = - \frac{3}{2}$

$\rm or, x = - \frac{3}{2} + 2$

$\rm or, x = - \frac{3 - 4}{2}$

$\rm or, x = \frac{1}{2}$

$\rm \therefore x = \frac{1}{2}$

Let this value of x be the value of $\rm x_2$. So, $\rm x_2 = \frac{1}{2}$.

Hence, the required coordinates of the ends of the latus rectum are $\rm \left ( \frac{7}{2}, \frac{15}{4} \right )$ and $\rm \left ( \frac{1}{2}, \frac{15}{4} \right )$

0